Edit to add: Don't worry about it, companies pay for people to be cleared all the time, sometimes they prefer to do it again anyway. Phone up DVA for more answers, they were quite helpful when I spoke to them.
I would assume that you'd have to be vetted again. If your contracting then the company your working for would take on your Clearance and the year period would start again. DVA are the best people to speak to!
Companies will only do it if your new job requires it, but as everyone's said, it will last a year from when you leave, so you have that grace. It's not much hassle if you lapse, and later find a job where you need it - the application goes in, though ironically it takes a few weeks longer to be processed than a ordinary decent civvie.